SOLVENT EXTRACTION PROCESS FOR PRODUCING LOW- NITRATE AND LARGE-CRYSTAL-SIZE PuO{11 {11 SOLS

ABSTRACT

Low-nitrate plutonia sols having a NO3/Pu mole ratio in the range 0.1 to 0.4 with an average crystallite diameter of 30 to 80 A can be produced when a sol is prepared by solvent extraction of a plutonium nitrate seeded with a plutonia sol. When the seeded sol is taken to dryness and heated for 10 to 120 minutes at a temperature in the range 180*-230* C. in a dry sweep gas, nitrate removal occurs and the baked solid can easily be dispersed to form a stable sol.

United States Patent [1 1 Lloyd 1 1 Feb. 26, 1974 [22] Filed: Aug. 30, 1972 [21] Appl. No.: 284,809

[52] US. Cl 423/251, 252/301.1 S [51] Int. Cl C01g 43/00 [58] Field of Search 252/301.1 S; 423/251 [56] References Cited UNITED STATES PATENTS 3/1967 Lloyd 423/252 X 11/1971 Haas et a1. 264/ 3,600,323 8/1971 Tallent 252/l.1 S 3,513,101 5/1970 Merservey 252/301.1 S 3,461,076 8/1969 Lloyd et a1. 252/301.1 S

Primary ExaminerCarl D. Quarforth Assistant Examiner-R. L. Tate Attorney, Agent, or Firm-Roland A. Anderson; John A. Horan; lrving Barrack [5 7] ABSTRACT Low-nitrate plutonia sols having a NO /Pu mole ratio in the range 0.1 to 0.4 with an average crystallite diameter of 30 to A can be produced when a sol is prepared by solvent extraction of a plutonium nitrate seeded with a plutonia sol. When the seeded sol is taken to dryness and heated for 10 to minutes at a temperature in the range 230 C. in a dry sweep gas, nitrate removal occurs and the baked solid can easily be dispersed to form a stable sol.

4 Claims, N0 Drawings SOLVENT EXTRACTION PROCESS FOR PRODUCING LOW- NITRATE AND LARGE-CRYSTAL-SIZE PUOz SOLS BACKGROUND OF TI-IE INVENTION The invention described herein was made in the course of, or under, a contract with the U. S. Atomic Energy Commission.

The present invention relates to plutonia sols. More particularly, it relates to a process for producing stable plutonia sols alone or as a mixed sol such as a plutoniaurania sol. Sols of the kind referred to in this invention are eminently useful as feed for the production of actinide oxide dense microspheres.

In the context of sol-gel technology for the production of oxide microspheres, a stable sol is one which has a useful shelf life and which is long enough to enable the sol to be processed 'to a microsphere'A sol with a useful shelf life means one which maintains a sol condition without settling out of the solid phase such as by precipitation or by gelling or which does not undergo any chemical change. Shelf life is important for production logistics, since it is generally desirable to maintain an inventory of sol available for processing into microspheres. To speak of a stable sol in terms of shelf life along, however, does not necessarily qualify the sol as stable since experience has shown that sols with long shelf life are not necessarily sufficiently operationally stable when they undergo processing to form microspheres, particularly where microsphere production involves mixed sols such as plutonia and urania.

The plutonia sols produced by this invention are derived principally from aqueous solutions of plutonium nitrate by a solvent extraction process known as the Alcohol-Plutonium Nitrate Extraction process and hereinafter referred to as the APEX process. The APEX process requires several steps, the first being extraction of nitric acid from an aqueous solution of plutonium nitrate with an aqueous immiscible alcohol, such as n-hexanol, to decrease the nitrate-to-plutonium mole ratio to about 2.5. The solution is then digested at elevated temperature, about 100 C., to hydrolyze the plutonium (which is thought to exist, by then in polymeric form), thus releasing more nitric acid. After cooling to room temperature, the solution or sol is further extracted with ri-hexanol to produce a sol with a nitrate-to-plutonium mole ratio of about 1.0. Alternately, the sol can be prepared by continuous extraction which avoids the digestion step, and this is the preferred method. Additional nitrate can be extracted to provide a final NO /Pu mole ratio of0.6-0.8 in a sol. Such a sol can be concentrated to more than I M in plutonium where the desired plutonium concentration is reached by evaporation. This procedure is referred to as the regular APEX process. It produces a stable, highly crystalline sol, where stability means a useful shelf life. Regular APEX sols are called high-nitrate sols, where high nitrate means a sol which has a NO lPu mole ratio in the range 0.6 to 1. High-nitrate sols have a limited utility for making PuO microspheres because of the deleteriously high nitrate content and high surface area of the solid phase of the sol which result in extensive cracking of calcined microspheres.

APEX sols have been found suitable for making PuO -UQ microspheres from mixture of an APEX so] with a U0, sol. There are, however, some severe shortcomings. Mixed sols of APEX-derived PuO and U must be maintained at about 45 C. in order to pro vide adequate shelf life for future processing to microspheres. Moreover, while a high-anion (nitrate) sol is satisfactory for shelf-life stability of PuO sols, high anion concentrations detract from the stability of mixed PuO -UO sols.

Static stability, as measured by shelf life, as well as operational stability of sols, has been associated with crystallinity as opposed to amorphous or polymeric solid phase of the sol, low nitrate (0.1-0.4) as opposed to high (0.6-1.0), and crystalline size. The crystallite size of urania in urania sols is inherently much larger than plutonia crystallites regardless of the sol preparation method, although these methods do not employ temperatures in excess of -60 C. The size of urania crystallites can also be additionally increased by digestion of the sol in the temperature range 100 C. Plutonia sols in general and APEX sols in particular consist of very small crystallites which show no detect able change in size when treated by any known method at temperatures below C. While it is possible to reduce the NO /Pu mole ratio of APEX sols by exhaustive extraction of concentrated sols at low temperature, there is no detectable crystallite growth, and the nitrate loss is accompanied by random aggregation of crystallites which is generally undesirable. Results from thermal denitration of sols prepared by a precipitation process show that high-nitrate sols require extensive baking periods at high denitration temperatures before an acceptably low No /Pu mole ratio is reached; and, while higher denitration temperatures will reduce the baking time necessary to reach a low-nitrate sol (i.e., with a No /Pu mole ratio of less than 0.2), very uniform heating is required. If overheating occurs, it is frequently impossible to redisperse the baked product back to sol form.

While high-nitrate plutonia sols prepared by solvent extraction procedures can be processed to gel microspheres, it has not been possible to obtain dense calcined microspheres with satisfactory yields. The principal problems involve particle malformation during the sphere-forming process, and a pronounced tendency for the particles to crack and disintegrate during form ing, drying, and calcining. The high nitrate content and high surface area of these plutonia sols also limit the ability to mix them with thoria sols since unstable mixtures frequently result. These problems associated with high-nitrate sols have been interpreted as resulting from high surface area and the high nitrate content which contribute to the physical and chemical instability of mixed sols and to the malformation experienced during the sphere formation process. Experienced investigators in this technology have long sought for a plutonia sol which is a low-nitrate sol and which contains large crystals of plutonia of the order of 50 A or more as opposed to a crystal size of 10-20 A on the reasoning that the larger crystals associate less nitrate due to the much lower surface area of the plutonia.

SUMMARY OF THE INVENTION With this background in mind, it is the principal object of this invention to provide a plutonia so] which has a useful shelf life and which can be subsequently processed to dense microspheroidal particles.

It is another object of this invention to provide a lownitrate plutonia sol which has a large crystallite size.

It is still another object of this invention to provide a plutonia sol which is both statically and operationally stable.

It is a further object of this invention to provide a method for realizing the aforementioned objects.

In its process aspect, the present invention may be regarded as an improvement in the APEX process involving two essential steps in which (1) a portion of a previously prepared APEX sol is added as a seed to a feed solution of plutonium nitrate as it undergoes nitrate extraction to form a sol and where (2) the resultant seeded sol can be converted by a baking operation in a controlled atmosphere to a low-nitrate plutonia sol with large crystallite size.

It has been discovered that, when a plutonium nitrate solution is seeded with a plutonia sol as the solution undergoes nitrate extraction, a distinctive ordered aggregation of particles, micelles, occurs which have been found, by electron microscopy techniques, to be as large as 150-200 A and are statically stable. Moreover, this aggregated micelle structure is not affected by sol aging at ambient temperature, by extensive nitrate extraction, or by boiling at 100 C. That is to say, the seeded" sol retains the sol form, but does not undergo crystallite growth. More specifically, seeded sols at this stage do not appear to afford any significant process advantage over regular APEX sols since both sols have comparable high NO /Pu mole ratios (0.6-0.8), and mixed UO -PuO sol stability is not improved by forming the aggregated micelle structure only. In other words, mixed sols of PuO and U derived from solvent extraction processes still need to be cooled to 4 C. to effect static as well as operational stability. However, the seeded sol with its aggregated micelle structure can be effectively denitrated to a low-nitrate sol and undergo crystallite growth when taken to dryness and heated. Specifically, low-nitrate sols (NO /Pu mole ratio of 0.l0.4) with average crystallite diameters (as measured by X-ray line broadening or electron micrograph techniques) in the range 30-80 A can be prepared when a sol, with the appropriate aggregated micelle structure induced by seeding, is taken to dryness and heated for to 120 minutes in a controlled atmosphere at temperatures in the range l80-230 C. When the seeded APEX sol is baked under thermal denitration conditions, a baked product is obtained which, upon redispersion with water, produces a lownitrate sol having crystallites in the range 30 to as high as 100 A. This is in direct contrast to the behavior of regular APEX sols which show no detectable change in crystallite size or any significant reduction in the NO /Pu mole ratio when heated to these temperatures for much longer times.

To form the product sol which is to serve as feed for microsphere formation, the baked product is mixed with water to effect redispersion of the solids into sol form.

The Seeding Effect Several variations in the seeding technique can be used to obtain the desired aggregated micelle structure. In the broad sense, the development of the desired micelle structure can be effected by introducing a plutonia so] during solvent extraction of a feed plutonium nitrate solution. Alternatively, and for reasons of consistency and reproducibility, it is preferred to use a parent seed sol which was itself prepared by a seeding technique. This is called double seeding.

A maximum micelle-forming effect can be achieved by multiple seeding by which is meant incremental addition of the feed nitrate solution during continuous extraction in a single run. A multiple-seeding effect is obtained because extraction is continued after each feed addition until plutonium polymerization occursand hence a sol is formed before the next addition of feed. Single-seeding experiments have been found to produce a so] in which an average micelle size of 50-75 A converts to an average crystallite size of 35-40 A in diameter. By the multiple-seeding effect average crystal lite sizes in the range 30 to a maximum of 100 A have been obtained after baking the seeded sol and redispersion of the baked seeded so].

The presence of the desired aggregated micelle structure can be detected by electron micrography and electron diffraction. In studies of non-seeded as well as seeded sols, the necessity of seeding has been established as the means for producing micelle structure in plutonia sols; and, by studying sols produced by redispersion of dried and baked sols, seeded or unseeded, the relationship between seeded sols has been established as the requisite source for low-nitrate and largecrystallite plutonia sols.

The amount of seed determines the existence and amount of micelle structure. A minimum of 20 percent of the total plutonium (seed feed) for single seeding and 25-30 percent for double seeding is required for effective development of the aggregated micelle structure. The NO -,/Pu mole ratio of the seed can be varied over fairly wide limits from 0.7 to 1.2. The age of the seed does not appear significant, provided only that it remains as sol and is not gelled. The seed polymer can be added to the feed (plutonium nitrate) solution prior to or during extraction, provided the conductivity of the feed is no less than about millimhos/cm. Poor results in terms of incomplete aggregation are obtained when seed is added to a feed having lower conductivity.

The concentration of plutonium in the aqueous feed solution can be varied over wide limits, from 15 up to 30 g Full of solution. However, at a plutonium concentration over 30 g/l, excessive plutonium extraction will occur as present techniques and the nitric acid concentration may range from 0.5 to 1.0 M. A concentration of 26 to 28 g Pu/l, 0.6 M HNO provides feed with an initial conductivity of about millimhos/cm.

Seeding (single, double, or multiple) may begin immediately upon APEX-type extraction of such solutions and continued until a No /Pu mole ratio in the range 0.7 to 0.9 is attained. Higher values appear to decrease the thermal denitration rate and lower values result in plutonia sols which will not resuspend after baking. APEX and similar solvent extraction processes are capable of reducing the NO /Pu mole ratio to a minimum of -0.6 even with a digestion step to produce a sol with good shelf-life stability. Further reductions in nitrate content by solvent extraction cause gelling or precipitation.

The aggregated micelle structure resulting from seeding promotes rapid denitration during baking. For example, a sol containing the desired aggregated micelle structure prepared by double seeding will denitrate to a No /Pu mole ratio of about 0.15-0.17 in 10 minutes at a baking temperature of 230 C. and will denitrate affecting the final sol in 30-40 minutes. For sols of this type, the micelles convert to an average crystallite size of about 60 A, and conversion to the larger crystallites appears to be essentially complete after -15 minutes of baking time. By comparison, an unseeded APEX sol with the same initial NO /Pu ratio will still be 0.4 after 4 hours of baking at 230 C.

A distinctively advantageous feature of this invention is that the dried sols are quite insensitive to overheating. The dried sols can be baked for extended periods even after reaching a prescribed NO /Pu condition without adversely affecting the characteristics of the sol formed by redispersion. On the other hand, unseeded plutonia sols (which do not have the aggregated .micellular structure) produced from solvent extraction or precipitation processes are exceedingly sensitive to baking time, leading to unpredictable final NO /Pu ratios and even to baked products which will not resuspend to sol. W 1

In order to reach a desired low-nitrate plutonia sol with large crystallite size, a baking temperature in the range l80-240 C. and preferably in the range 200-230 C. was employed to produce a baked product which is easily redispersable to the final stable product sol. In a typical cycle, the APEX sol or other sol derived from solvent extraction is'evaporated to dryness in air, rapidly heated to the desired baking temperature, and then held at the selected baking temperature for a period of 10 to 60 minutes. The baked product is allowed to cool to room temperature and then redispersed to form the desired sol.

Having described the invention in general terms, specific embodiments are now provided which illustrate in specific examples how the invention may be practiced to produce plutonia and mixed plutonia-urahia sols of enhanced stability.

I EXAMPLE 1 Preparation of the Seed Sol To prepare seed material, 500 ml of plutonium nitrate feed solution (Pu 26 to 28 g/l; HNO =0.6-0.8 M.) was contacted with about 3 liters of n-hexanol in the APEX extraction equipment. The initial conductiv-. ity of the feed solution was 100 millimhos/cm. The feed solution was extracted continuously at 25 C. until the conductivitywas reduced to 6 millimhos/cm which cor responds to a NO /Pu mole ratio of 0.8 i 0.1. The sol was removed from the extraction equipment and concentrated by evaporation to 150-200 ml. This concentration step is made only to provide a convenient volume for subsequent handling and does not affect the properties of the sol.

EXAMPLE 11 Preparation of Single-Seed Sol tonium in the system. The extraction was continued at 25 C. until the conductivity of the so] was 6 millimhos/cm corresponding to a No /Pu mole ratio of 0.8 i 0.1. The sol was removed from the extraction vessel and concentrated to a convenient volume (-l50 ml) by evaporation.

EXAMPLE [[1 Preparation of Double-Seed Sol Five hundred ml of plutonium nitrate feed solution (Pu=26-28 g/l; HNO =0.60.8 M) was charged to the extraction equipment and contacted with 3 liters of n-hexanol at 25 C. until the conductivity of the feed solution was reduced to millimhos/cm. Seventy-nine (NO /Pu mHe ratio= 0.8 i 0.1). The sol was then evaporated to dryness and baked. The sol was taken to dryness at 80-90 C. and the temperature was not allowed to exceed C. The dry mass was then charged to a baking apparatus consisting of a stainless steel pan in an aluminum block containing a heating element. The baking apparatus was preheated to 230 C. before the solids were added. During baking, a lid was placed over the pan and air was pulled through the baker to remove gaseous decomposition products. Baking times of 15, 40, and 75 minutes at temperature resulted in NO /Pu mole ratios of 0.16, 0.13, and 0.10,

respectively, where the NO /Pu concentrations were measured in the sol resulting from dispersion of the baked mass. The sol can be heated for at least an additional 60 minutes without reducing the No /Pu mole ratio or otherwise noticeably affecting the characteristics of the sol. The solids are then removed from the baking apparatusand suspended in water. Excess water can be used if desired and the sol can be concentrated to greater than one molar by evaporation.

EXAMPLE IV Single-Run Multiple-Seeding Sol Preparation Fifty ml of seed sol (116 mg Pu/ml) was mixed with 50 ml of plutonium nitrate feed (Pu= 21.5 m'g/ml;

, HNO 0.6 M) and extracted with 3 liters of n-hexanol until the conductivity was 25.5 millimhos/cm. Two

hundred ml of feed was added and extraction was continued until a final conductivity of 6 millimhos/cm was obtained. The extraction was continuous, and the total extraction time was about 30 percent less than that required in the experiments described abovehThe sol was then removed from the extraction equipment, evaporated to dryness, and baked as previously described. The final sol was essentially identical to the sol obtained by double seeding.

Thesols formed in accordance with the preceding examples produce a low-nitrate sol derived from the baking procedure with enhanced stability and larger crystallite sizes than have been possible by other methods of which we are aware. The effect of drying temperature on seeded as well as unseeded sols is shown in the following table.

and 200 C.) also showed micelle aggregates present.

However, a change in the electron diffraction patterns THE APEX PROCESS Approximate average NOs/ Pu crystalline Approximate mole ratio size of PuO Mixed UO -PuO drying temp. of unmixed in P1102 sol sol shelf life Sol preparation (C.) PuO sol (A) at 25 C.

Regular APEX Not dried 0.71 10-20 30 min. Regular APEX.. 200 .63 1020 1 hr. 45 min. Regular APEX 230 .68 10-20 1 hr. 45 min. Continuous extraction APEX Not dried .67 10-20 1 hr. 50 min. Continuous extraclion AEX 200 .68 10-20 10 hr. Single seeding continuous extraction..... 180 .53 20 hr.

Do 230 .39 30-40 132 hr. (5 /2 days). Double seeding continuous extraction... Not dried .64 -20 1 hr.

160 .63 2 hr. 30 min. 180 .50 10 hr. 200 .41 48 hr. Do 220 .20 55'65 138 hr. Multiple feed addition 230 .19 55-75 192 hr. (8 days).

*Denotes continuous extraction of plutonium nitrate solution with n-hexanol down to a NO Pu mole ratio of 1.0 without digestion at 100 C.

Consists generally of both aggregates of small crystallites (10-20 A) and 30-40 A crystallites which resulted from conversion of aggregates.

***Consists of both aggregates of small (10-20 A) and 55-75 A crystallites.

As seen from the table, drying temperatures in the range l60-230 C. were used. When sols were dried at the higher temperatures, they were evaporated to dryness slowly at temperatures below 100 C. The dry solids were then rapidly heated to the desired temperature, held at that temperature for 30 minutes, removed from the heat, and allowed to cool. Sols which were not dried wereconcerit ra ted to 1 M PuO by evaporation and extracted three times with n-hexanol. Ten volumes of n-hexanol were contacted with one volume of sol during each extraction. This procedure was expected to remove all of the extractable HNO Mixed-sol stability tests were made by mixing sufficient PuO sol with CUSP-prepared U0 sol (which had been freshly contacted with an equal volume of water-saturated nhexanol) to provide a total metal concentration of 0.95 M (80 percent U6 percent PuO and the S015 were aged at C. until gelation occurred.

Electron micrographs and electron diffraction patterns were obtained for all of the sols shown. There was no apparent change in crystallite size or crystallinity for the unseeded sols when they were dried at 200-230 C. The No /Pu mole ratios for unseeded sols were not affected by this heating step, indicating that there was no appreciable change in crystallite size. Seeded sols showed a regular decrease in NO /Pu mole ratio with increasing drying temperature, with a corresponding increase in average crystallite size. This increase in size was apparent from both electron micrographs and electron diffraction patterns of the dried sol. The micrographs indicated that the increase in average crystalliate size with increasing drying temperature does not result from gradual growth of small crystallites, but from an increasing number of micelle aggregates which have converted to larger crystallites. For example, in the double-seeding runs shown, a micrograph of the undried sol indicated that very small crystallites (IO-20 A) are primarily aggregated to form micelles which vary in diameter from 50 A to -180 A. Micrographs at the intermediate drying temperatures (160 180,

for material heated at and 200 Ciindicated that large crystallites are also present, correlating with the decrease in NO /Pu mole ratios shown in the table. At 200 C., a rather dramatic change in the appearance of the micrograph occurred because the micelles had been completely converted to larger crystallites. This complete conversion was also indicated by the electron diffraction pattern and the NO lPu mole ratio.

These changes in sol characteristics are also reflected by the improvements in mixed UO -PuO sol shelf life at 25 C., as shown in the right-hand column of the table. The data reflect that a substantial increase in shelf life is obtained even before complete conversion of the micelles to crystallites occurs; that is, with sols baked in the 180220 C. range. a

One very important feature of obtaining relatively large crystallites by the seeding-and-baking technique is that the dried sols appear to be quite insensitive to overheating. It has been found, for example, that seeded sols could be heated much longer than neces sary for optimum crystallite formation without adversely affecting final sol characteristics.

The Effect of Denitration Atmosphere on Obtaining a Stable Resuspended Sol It has been found that denitration atmosphere affects the denitration rate as well as the redispersion quality of the baked product. When air was used as the sweep gas, less than. 15 minutes at 230 C. was required to obtain a desirable 60-A-crystallite -size final sol of adequately low nitrate content, whereas 50 minutes was required to reach the same condition in a stagnant atmosphere. Similar results were obtained when argon, nitrogen, or oxygen was used as sweep gas.

One of the most startling and unexpected observations made in the denitration tep is the effect of water vapor on the redispersion quality of the baked product. This effect is demonstrated in the following example.

EXAMPLE V Two high-nitrate sols having a NO /Pu mole ratio 06 were denitrated under different sweep gas conditions. The first sol was heated to dryness'and then baked at a temperatue of 230 C. for about minutes in a dry argon atmosphere having a water vapor content of less than 4 parts per million parts of sweep gas. The baked product was then mixed with water to formulate a plutonia sol have a No /Pu mole ratio of 0.2 with' a solid phase containing crystallites averaging 60 A in size. Sols obtained after heating over 4 hours at temperature were readily reformulated to a sol of the same quality. The sols were dark green to black in color and exhibited a high degree of static stability.

In a second experiment, the second high-nitrate sol was dried and baked for 20 minutes at 230 C. in a wet argon atmosphere; that is, an atmosphere in which dry argon sweep gas was bubbled through a water trap at C. to introduce water vapor. As in the first experiment, conversion to 60-A crystallites occurred within about 20 minutes. However, the sol resulting from addition of water to this baked solid product to form a NO /Pu mole ratio of 0.2 was very light green in color. Solids resulting from heating for minutes in a wet argon atmosphere could not even be resuspended to form a sol. These and similar experiments indicate that the sol-forming quality of the baked product is extremely sensitive to overheating when a wet sweep gas is used and quite insensitive when a dry sweep gas is used.

From these experiments and measurements of crystallite size obtained by electron microscopy and X-ray line broadening which show that both dry and wet sweep gases produce nearly equal crystallite growth, I have deduced that crystallites from high-nitrate seeded or unseeded plutonia sols will undergo one and, in some cases, two growth mechanisms during thermal denitration at elevated temperatures, i.e., l80240 C. depending on the water content of the sweep gas. With a dry sweep gas, only crystallite growth occurs and this leads to stable sols. With a wet sweep gas, desired crystallite growth is accompanied by an irreversible, random aggregation of crystallites which is not desireda condition which produces large particles which are extremely difficult to resuspend to form a sol. This large particle formation is readily detected by the color of the sol. When the crystallites are primarily unassociated, the resuspended sol is very dark green, nearly black. As larger particles form, the color of the sol becomes a much lighter green. In a severe case, the so] is nearly white and the solids settle rapidly upon standing. On the other hand, sols formed from baking the dried solid phase ina dry sweep gas form extremely stable sols and show a complete absence of settling or segregation during storage. Once the effect of water vapor in producing a resuspendable baked solid to form a stable sol was determined, reproducible results were routinely attained. Therefore, when I refer to the use of a dry sweep gas, I mean a sweep gas which leads only to the desired crystallite growth in the baked product, and

a wet sweep gas defines a water vapor content in the sweep gas which leads to both crystallite growth and the undesired, adverse, random aggregation of crystallites. In general, a sweep gas which is dry in the sense of and for the purposes of this invention should contain no more than parts of water vapor per million parts of sweep gas. Greater amounts will tend to produce a wet effect.

The water vapor effect is independent of the way sols are initially prepared. That is to say, dried, baked products resulting from sols prepared by precipitation techniques, by solvent extraction, or by seeding will be similarly affected by adverse amounts of water vapor in the sweep gas. Of the three techniques, seeding is the preferred way to form the initial sol because the denitration rate of such sols is extremely insensitive to baking temperature without affecting the dispersability of the solid phase or the stability of the resulting sol.

In summary, it has been shown that low-nitrate sols can be prepared (NO /Pu mole ratio 0.1-0.2) with average crystallite diameters of -60 A when a seeded sol with the appropriate aggregate structure is taken to dryness and heated for l0 to 60 minutes at a temperature in the preferred range of 200230 C. While lower temperatures may be used down to l80 C., the denitration times become excessively long. This is in direct contrast to the behavior of unseeded APEX sols, which show no detectable change in crystallite size or any significant reduction in the NO /Pu mole ratio when heated at these temperatures for much longer times. The specially prepared sols with low nitrate-toplutonium mole ratios and with larger crystallites exhibit greatly improved mixed-sol shelf life. And finally, it has been shown that the water content of the sweep gas affects the resuspension qualities of the baked solid to form a stable sol as evidenced by the dark green to black color of the sol as opposed toa white to light green color.

What is claimed is:

l. A method of preparing a plutonia sol which comprises mixing a seed plutonia sol with an aqueous solution of plutonium nitrate, extracting nitrate from the resulting mixture until a sol having a conductivity in the range 5 to 7 millimhos correspondingto a NO /Pu mole ratio in the range 0.7 to 0.9 is reached, drying the resulting sol to solid, heating said solid in a dry sweep gas to effect denitration and crystallite growth, and then dispersing the baked product to form a low-nitrate sol having an average crystallite size in the range 30 to l00 angstroms.

2. The method according to claim 1 in which the amount of seed sol is sufficient to produce an aggregated micellular structure resulting from nitrate extraction of said plutonium nitrate solution.

3. The method according to claim 1 in which the seed sol is itself derived from the nitrate extraction of a solution of plutonium nitrate.

4. The method according to claim 1 in which the dried sol is baked at a temperature in the range 180. to 240 C. for a period of IO to minutes. 

2. The method according to claim 1 in which the amount of seed sol is sufficient to produce an aggregated micellular structure resulting from nitrate extraction of said plutonium nitrate solution.
 3. The method according to claim 1 in which the seed sol is itself derived from the nitrate extraction of a solution of plutonium nitrate.
 4. The method according to claim 1 in which the dried sol is baked at a temperature in the range 180* to 240* C. for a period of 10 to 120 minutes. 